This invention relates generally to adaptive equalizers for baseband and carrier modulation systems such as single sideband and quadrature amplitude modulation communication systems, and more particularly to a constrained adaptive equalizer for use in eliminating amplitude and delay distortion in a digital signal.
In digital data transmission systems, the presence of amplitude and delay distortion in the transmission medium gives rise to intersymbol interference (ISI). Intersymbol interference and noise result in errors in determining the value of the transmitted signal at the receiver. The effect of intersymbol interference is to increase the likelihood that any noise in the system will lead to such error.
Adaptive equalizers have been used to reduce intersymbol interference. All methods and devices for adaptively equalizing the digital signal proposed thus far produce some noise amplification. It is noted that these problems of noise and distortion exist in both baseband and passband communication systems. Further, it is noted that adaptive equalization has been used to suppress distortion in both types of systems.
In the development of equalization techniques, initially the structures proposed for both baseband and passband were pure transversal equalizers. These equalizers adequately suppress ISI but may excessively amplify system noise. These transversal equalizers have leading and lagging taps (multiplying coefficients) for operation or precursors (leading echoes) and postcursors (trailing echoes) of the received signal, respectively.
In order to reduce this additional noise penalty, it has been proposed, both in the baseband and passband, to equalize only the precursors with a transversal equalizer and the postcursors with a decision feedback equalizer. This structure necessitates only half the number of stages of the transversal equalizer, i.e., a "one-sided" transversal equalizer, thereby reducing the noise penalty. Since the decision feedback equalizer only subtracts from its input signal a set of distortion components, and since these distortion components are derived from decided data signals and are therefore noise-free, there is no further noise amplification. Nevertheless, the amount of noise attributable to the one-sided transversal equalizer will still, under certain transmission conditions, be excessive.
Another approach to reducing noise amplification was proposed by Lyon in an article entitled "Timing Recovery and Synchronous Equalized Data Communication", IEEE Transactions on Communication, February, 1975, pp. 269-274. Lyon recognized that nulls in the spectrum of the sampled received signal could be created by the process of sampling. The attempt to equalize such nulls leads to severe amplification of noise. Lyon suggests sampling at a time epoch chosen to minimize nulls and thus minimize the noise amplification. This proposed system was very complex to implement, especially the timing recovery circuit. It required that a special training sequence be known at the receiver to guarantee equalizer convergence. This pure transversal system with special timing still resulted in excessive noise.
Qureshi and Forney, in an article entitled "Performance and Properties of a T/2 Equalizer", NTC, 1977, 11:1, proposed a fractional tap spacing transversal equalizer. This equalizer avoids generating nulls in the sampled spectrum by sampling at twice the baud rate. One disadvantage of this approach is that digital signal processing must take place at a higher speed than that required by the baud rate sampling. Thus, long equalizers are required, and the noise penalty for amplitude distortion on the channel is still exacted.
The systems utilizing one-sided transversal equalizers recognize the presence of nulls in the sampled spectrum. By incorporating the decision feedback equalizer for the postcursor correction the effect of noise amplification from equalization of all of the nulls in the sample spectrum is reduced.